The present invention relates to power steering systems for use with automobiles and the like and more particularly to a power steering system which has excellent steering feeling.
In the case of automotive vehicles or the like whose user classes cover many divergencies, presently the provision of power steering is the practice with vehicles of large and small sizes and power steering is useful in reducing the fatigue of the driver and thereby ensuring safe driving.
The power steering systems heretofore known in the art have been mainly of the hydraulically operated type. In recent years, however, electrically-operated power steering systems have been proposed by noting their substantial control contents and merits from the standpoint of energy saving.
Of these electrically-operated power steering systems, those which have been used mainly are of the type in which an electric motor is employed as an electrically-operated actuator so as to produce a large operating force required for the actuator and its output is slowed down to obtain a final operating force for steering assisting purposes.
In the conventional electrically-operated power steering systems, the assistant steering force is provided by the electric motor operated at a high speed through the reduction mechanism. On the other hand, the electric motor involves an inertial resistance due to the mass of the armature (rotor) and also the reduction mechanism includes a train of gears thereby causing a large mechanical resistance in turning the output shaft of the reduction mechanism. Therefore, in the case of FIG. 1, there is a disadvantage that if a motor 7 is not energized, there is no action of any assistant steering force so that, due to the previously mentioned inertial resistance and mechanical resistance, a large force is required for turning a steering wheel 1, that is, the steering wheel 1 becomes heavy and the steering wheel restoring performance is deteriorated thus causing characteristics which are quite undesirable from the standpoint of steering feeling to appear in the operation of the steering wheel.
As regards these undesirable characteristics, particularly those undesirable characteristics from the standpoint of steering feeling which manifest in the operation of the steering wheel due to the inertia of the electric motor, as proposed for example in Japanese Laid-Open patent application No. 55-76760, etc., it is conceivable to use a method of providing a differential characteristic in the control of the electric motor and this method is expected to have a considerable steering feeling improving effect. With this method, however, the provision of an additional differentiation circuit is required so that the provision of the differential characteristic further requires another consideration with respect to noise thus tending to increase the cost and no improving effect is expected with respect to the deterioration of the steering feeling due to the previously mentioned frictional resistance. Thus, on the whole, this method is not capable of improving the steering feeling satisfactorily. It is to be noted that the deterioration of the steering feeling due to the frictional resistance takes the form of a decrease or complete loss of the restoring force which should essentially be produced at the steering angles other than the zero steering angle (neutral position).